We are overlooking one of the streets there for the main course, like a 100 (metres) of the finish and people started running by the front and then you started to get a bit worried."

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6:37am: It's unclear what's happening at the JFK Library: the Library itself has tweeted this:Fire in building is out, appears to have started in the mechanical room of new building. All staff and visitors are accounted for and safe

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- @JFKLibraryInvestigators are investigating. Any tie to Boston Marathon explosions is pure speculation. More information as we receive it

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- @JFKLibraryAustralian mining magnate Clive Palmer is a board member at the library

.6:34am: BREAKING: Reuters is reporting another explosion at Boston's JFK Library, south of the CBD

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Boston police confirms another explosion at JFK Library

- @ReutersUS6:32am: Australian Jeff Hunt, who finished in the top 15 along with Lee Troop, has spoken to ABC News Breakfast

."I was in my hotel room when I [heard] what turned out to be an explosion which actually went off about quarter to 3, I think. I came through, we got ushered out. All the elite athletes were in the hotel when all this went off, and basically ended up having a Twitter explosion and seeing things going down and finding out that a couple of bombs have gone off and we're in lockdown

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"We're staying about 200, 300m from the finish line and the bombs went off about 200, 300m off the finish line, so just as the bulk of the 20,000 were coming through.Â

"[I] had finished, [was] going through the recovery, had come upstairs, had a shower and I heard it go off. I didn't think much thought of it. I thought that something big had dropped or fallen over or something like that. I've never heard an explosion at all in my life.Â

"New York I naturally - I thought it was something else. I mean, I wouldn't think that anyone wouldn't think that anyone would want to do something like that. I mean, I'm an optimist, and it was such a great day, everything was perfect and the whole city was in good spirits, everything was going really, really well, so that thought never ever entered my mind."

6:28am: US president Barack Obama has been told about the Boston blast. There are also reports of heightened security around the White House.

A White House official says the US President, Barack Obama, has been notified of the "incident in Boston."

- Shelley LloydDFAT says Australia's Consulate-General in New York is making urgent enquiries to find out if any Australians have been hurt

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Dept of Foreign Affairs spokeswoman: "At this time we are not aware that there are any Australian victims"

- ABC reporter Simon Cullen6:26am: Video showing the moment of the first explosion

.6:25am: Steve Moneghetti says all the Australians he knows at the marathon are OK

- @steve_mona6:21am: The Associated Press, citing an intelligence official, says two explosive devices have been found at the Boston Marathon

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MORE: US official: 2 more explosives found at Boston marathon

- @AP6:15am: US media is reporting a small home-made bomb was the cause of at least one of the explosions. No official confirmation yet

.6:13am: Former Australian marathon champion Rob De Castella is in Boston and is trying to find a Darwin woman running in the event

.Emma Cameron was competing in the marathon as part of the Indigenous Marathon Project

.Mr De Castella believes Ms Cameron is OK because she was a long way from the finish line when the explosions happened

.6:07am: Boston Police have confirmed two deaths

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Update 23 injuries 2 dead

- @Boston_Police6:02am: Boston police have called in all their off-duty officers. There are no confirmed fatalities at this stage

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CNN is reporting that 19 people are being treated at Massachusetts General Hospital.

6:00am: The Boston Mayor's office says police have confirmed the explosion:Please avoid area of Copley Square to allow first responders to work

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- Mayor Thomas Menino's Office

US reporter Jackie Bruno is at the scene and says she and her cameraman felt the force of the explosion:I saw people's legs blown off. Horrific. Two explosions. Runners were coming in and saw unspeakable horror

.The explosions took place about three times after the end of the marathon

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5:51am:There were two bombs that exploded near the finish line in today's Boston Marathon. We are working with law enforcement to understand what exactly has happened

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- @bostonmarathon

5:49am:

Only thing confirmed at the moment re Boston is that there were two loud explosions at finish line of marathon and the scene is chaotic

- ABC correspondent @LisaMillar

5:46am:Trauma nurse with Mass. General Hosp. says there are possibly two fatalities and some victims with severed limbs

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- @KSATnews

5:40am: Eyewitness Mike Mitchell of Vancouver, Canada, said he was looking back at the finish line and saw a "massive explosion."Smoke rose 15 metres in the air, Mitchell said. People began running and screaming after hearing the noise